Wattpad, a Union Square Ventures-backed platform for sharing stories and interacting with writers, has been growing steadily, and it hit a nice milestone in January — during that month, users spent more than 1 billion minutes on the service.

Co-founder and CEO Allen Lau tells me via email that the growth comes from a combination of attracting new users and convincing each of those users to spend more time on the site. The average session for Web users was 30 minutes, while visitors on Android devices spent an average of 28 minutes.

“We are relying more and more on new users now,” Lau says. Given the Android stats, in particular, “There is simply not much room to grow further (we only have 24 hours per day).”

And not surprisingly, the majority of that activity is happening on mobile devices. Of those 1 billion minutes, Lau says that about one-third came from the Web, while another third came from Android, 20 percent came from iOS devices, and the rest comes from other mobile platforms like BlackBerry and Windows Phone.

In announcing the numbers, Wattpad also emphasizes the sheer quantity of content being shared. There were 300,000 stories uploaded in January (the site has 3.2 million total), and given the average word count per story, that adds up to more than 4 billion words — the equivalent of 8,856 Lord of the Rings trilogies.